简单的写法

#[allow(dead_code)]
fn output(filename: &str, bytes: &[u8]) -> Result<()> {
    let mut fp = OpenOptions::new().truncate(true).create(true).write(true).open(Path::new(filename)).chain_err(|| format!("fail to open {}", filename))?;
    fp.write_all( bytes )?;
    fp.write_all( &['\n' as u8] )?;
    fp.flush()?;

    Ok(())
}

或者，如下写法更有 Rust 的感觉

#[allow(dead_code)]
fn output(filename: &str, bytes: &[u8]) -> Result<()> {
    OpenOptions::new().truncate(true).create(true).write(true).open(Path::new(filename)).and_then(|mut fp| {
        fp.write_all( bytes )?;
        fp.write_all( &['\n' as u8] )?;
        fp.flush()?;
        
        Ok(())
    })
}

引入 BufWriter， 增加写缓冲

#[allow(dead_code)]
fn output(filename: &str, bytes: &[u8]) -> Result<()> {
    let fp = OpenOptions::new().truncate(true).create(true).write(true).open(Path::new(filename)).chain_err(|| format!("fail to open {}", filename))?;
    let mut writer = BufWriter::with_capacity( 1024*1024*4, fp );
    writer.write_all( bytes )?;
    writer.write_all( &['\n' as u8] )?;
    writer.flush()?;

    Ok(())
}

当 filename == '-' 时，写到标准输出（STDOUT）

#[allow(dead_code)]
fn output(filename: &str, bytes: &[u8]) -> Result<()> {
    let fp = match filename {
        "-" => Box::new(stdout()) as Box<Write>,
        filename => {
            let path = Path::new(filename);
            //let fp = OpenOptions::new().append(true).create(true).write(true).open(path).unwrap();
            let fp = OpenOptions::new().truncate(true).create(true).write(true).open(path).unwrap();
            Box::new(fp) as Box<Write>
        },
    };

    let mut writer = BufWriter::with_capacity( 1024*1024*4, fp );
    writer.write_all( bytes )?;
    writer.write_all( &['\n' as u8] )?;
    writer.flush()?;

    Ok(())
}